Ubuntu and Kubuntu are basically the very same distribution, only they use a different desktop environment (and selected applications for that desktop environment to do your daily tasks - office, media player, whatever). Therefore, as far as I can tell having used both for quite some time:
Ubuntu with KDE = Kubuntu,
Kubuntu with Gnome = Ubuntu
if you also switch the applications that come with the desktop (why would you want to have Gnome apps in KDE or vice versa, making it look unnatural?).
The reasons you don't get to select packages in the installer are, in my point of view, these:
- the installer actually can fit on one cd only (small download size, happy user)
- Ubuntu's ideology is not to install seven apps for one need, but try to pick one good for one need; hence no need to ship with seven media players, if one can do the job
- because desktops seem to matter, it could be stupid to ship with only one desktop "right out of the package" - that's why there are Kubuntu (for KDE lovers) and Ubuntu (the "original", for Gnome people)
- you can freely install, remove and change software after the initial installation using Synaptic (or even manually with dpkg)
So, after you've installed either one, you can add the other desktop by selecting it in Synaptic. A word of warning: your menu entries will probably get mixed, having Gnome apps in KDE menu and vice versa, so you're going to have to clean them up. In addition, especially in the long run, some weird problems can rise: an example of my own is having an Ubuntu system throw errors of KDE programs, and have (at least seemingly) KDM shut the thing down, even though GDM is used instead. However it's possible to switch your desktop, or have both of them, or have XFCE (as in Xubuntu), and the horde of window managers out there. Yeah, it's completely possible - but if you want them all out of the box, at the same time, you're not asking for the "Ubuntu way of doing it", you're asking for a Fedora DVD (or even two in the future if it keeps growing).
You can also switch from Kubuntu to Ubuntu or vice versa by removing/purging and installing kubuntu-desktop vs. ubuntu-desktop using Synaptic. Those two are metapackages (they don't contain any files themselves, but depend on other packages) so they'll drag in the needed packages for Ubuntu-desktop or Kubuntu-desktop. Simple as that. Only Debian's package management system, apt, may confuse things